In recent years, especially in business offices, "desktop publishing", i.e., document creation using a computer is widely spread, and a demand has been increased recently for outputting not only characters and graphics but also a color natural image such as a photograph together with characters and graphics. In order to answer such a demand, it has become necessary to print out a natural image of high quality requiring reproduction of halftones.
Moreover, a so-called on-demand type printer is being rapidly spread. This is a printer, in which an ink droplet is discharged from a nozzle and applied to a medium such as a paper and a film only when necessary during a printing. Such a printer has a possibility to be reduced in size and cost.
For discharging an ink droplet, various methods have been suggested. Among them the most popular method employs a piezoelectric device or a heating device. The former is a method for discharging ink by applying a pressure to the ink by deformation of the piezoelectric device. The latter is a method for discharging ink by pressure of foams generated in the ink heated to boil by the heating device.
Also, there have been suggested various methods for reproducing the aforementioned halftones by using the on-demand type printer which discharges the aforementioned ink droplet. As a first method, the voltage level or pulse width of the voltage pulse to be applied to the piezoelectric device or the heating device is changed so as to control the size of the droplet to be discharged and to change the diameter of a printed dot.
However, this method has a problem that if the voltage level or pulse width to be applied to the piezoelectric device or the heating device is decreased too much, ink discharging is disabled. Consequently, the minimum droplet diameter has a limitation, decreasing the number of gradation steps which can be expressed and disabling expression of a low concentration. That is, this method is insufficient for printing out a natural image.
A second method does not change a dot diameter but employs a pixel composed of a matrix of, for example, 4.times.4 dots. Gradation expression is realized on this matrix base by using a so-called dither method. In this case, it is possible to express 17 gradation steps.
However, this method also has a problem. For example, if a printing is carried out with the same dot density as in the first method, the resolution is decreased to 1/4 of the first method, and only a rough image can be obtained. That is, this method is also insufficient for printing out a natural image.
In order to eliminate these problems, the inventors of the present invention have suggested a printer in which ink is mixed with diluent when discharged, so as to change the concentration of the discharged ink droplet, enabling to control the concentration of a printed dot. That is, the printer is able to print out a natural image without deterioration of the resolution.
Such a printer comprises a printing head having a first nozzle into which a discharge medium is introduced and a second nozzle which is provided adjacent to the first nozzle and into which a quantitative medium is introduced so that a predetermined quantity of the quantitative medium seeps out from the second nozzle toward the first nozzle so as to be mixed with the discharge medium in the vicinity of an opening of the first nozzle; the discharge medium is pushed out from the first nozzle together with the discharge medium which has been mixed with the quantitative medium; and the quantitative medium and the discharge medium are discharged in a direction contained in a plane determined by the first nozzle and the second nozzle. In such a printer, by changing the quantity of the quantitative medium which is either ink or diluent, it is possible to change a mixing ratio of the ink and the diluent so as to change a dot concentration, enabling to print out a natural image. It should be noted that the quantitative medium may be either ink or diluent and the discharge medium may be the remaining one.
As has been described, in the printer which mixes ink with diluent to be discharged, it is necessary to accurately control the mixing ratio of the ink and the diluent so as to accurately express a gradation step in accordance with an image data. In order to achieve this, it is necessary that the ink be sure to be separated from the diluent during a wait state, i.e., when no mixing of the ink with the diluent is carried out. If the ink is in contact with the diluent during the wait state, the ink flows into the nozzle into which the diluent is introduced and the diluent flows into the nozzle into which the ink is introduced. This adversely affects the mixing ratio of ink and the diluent in a following dot, disabling to express an accurate gradation step and accordingly, to obtain a recorded image of a high resolution.
Consequently, in such a printer in which ink is mixed with diluent to be discharged, it is desirable that at least a region sandwiched by the opening of a quantitative nozzle and the opening of a discharge nozzle have a property of liquid repellence.
Also, in the aforementioned printer which discharges ink alone, if the ink is attached around the opening of a discharge nozzle, the discharge direction becomes unstable and it becomes difficult to form a recorded image of a high resolution. Consequently, it is preferable that a region around the nozzle opening have a property of liquid repellence. The same applies to the aforementioned printer which mixes ink with diluent to be discharged.
As a material having such liquid repellence, there can be exemplified polytetrafluoroethylene or the like, and such a material is used for the region around a nozzle opening in the printer as has been described.
On the other hand, when manufacturing such a printer, nozzle formation is usually carried out by means of ablation processing using eximer laser.
However, the aforementioned polytetrafluoroethylene cannot be subjected to ablation processing by eximer laser. To cope with this, as disclosed in Japanese Patent Laid-Open Hei 6-328698, a material which absorbs light having a wave length of eximer laser is dispersed in the polytetrafluoroethylene, thus enabling to apply ablation processing by eximer laser so as to form a nozzle. However, if the method disclosed in the aforementioned Japanese Patent Laid-Open Hei 6-328698 is employed, there arises a problem that either the property of liquid repellence or the property of processability with eximer laser should be sacrificed to a certain degree, i.e., it is difficult to obtain both of the properties simultaneously to a full extent.
It is therefore an object of the present invention to provide a printer which enables to assure the liquid repellence around a nozzle opening so as to form a recorded image of a high resolution and which enables to form a nozzle by means of ablation processing using eximer laser, thus bringing about a preferable productivity.